Warp tension control mechanism

ABSTRACT

To maintain constant the tensions in warp yarns fed from a yarn beam of the negative let-off motion type of a loom, an improved warp tension control mechanism is herein proposed which consists of a brake unit adapted to exert a substantially constant braking torque on the yarn beam and a brake control unit which is responsive to variation in the tensions in the warp yarns being fed from the yarn beam and which is operative to lessen the braking torque on the yarn beam in response to an increase in the tensions in the warp yarns as caused by the stepwise reduction of the outside diameter of the roll of the yarns on the yarn beam and by any unforeseen operational conditions of the loom during weaving operation.

I Umted States Patent 1191 1111 3,860,044

Mizuno Jan. 14, 1975 [54] WARP TENSION CONTROL MECHANISM 3,750,715 8/1973 Mizuno 139/110 [75] Inventor: Yukio Mizuno, Tokyo, Japan FOREIGN PATENTS OR APPLICATIONS 73 Assignee; Ni Motor Company, Ltd 804,784 4/1951 Germany 139/100 Yokohama, Japan 886,729 8/1953 Germany 139/100 [22] Filed: 14, 1973 Primary ExaminerJames Kee Chi [21] Appl. No.: 332,331

[57] ABSTRACT [30 Foreign Appncation p i i Data To maintain constant the tensions in warp yarns fed Feb 15 1972 Japan 4745918 from a yarn beam of the negative let-off motion type July' lol972 Japan 47 68820 of a loom, an improved warp tension control mechanism is herein proposed which consists of a brake unit [52] CL 139/109 adapted to exert a substantially constant braking [51] Int CL 49/08 torque on the yarn beam and a brake control unit [58] Field 'g' 139/109 1 100 which is responsive to variation in the tensions in the 66780 warp yarns being fed from the yarn beam and which is operative to lessen the braking torque on the yarn [56] Reierences Cited beam in response to an increase in the tensions in the warp yarns as caused by the stepwise reduction of the UNITED STATES PATENTS outside diameter of the roll of the yarns on the yarn 2,397,663 4/1946 Hepp 66/86 A b am and by any unforeseen operational conditions g m of the loom during weaving operation. eer 3,072,154 1/1963 Locher 139/110 8 Claims, 3 Drawing Figures F111: 59 54 l 38 l PATEHTED 3.860.044

sum 1 or a PATEN JAN 1 4197s SHEET 2 OF 3 g o ow 6 7 cm ve I! 0 III llll ll 0 i 8 7 m Y. a s Q & w

WARP TENSION CONTROL MECHANISM The present invention relates to looms and, more paritcularly, to a .warp tension control mechanism for use in a loom of a specific character having a warp yarn supply means of the negative let-off motion type.

In the loom using the warp yarn supply means of the negative let-off motion type, warp yarns are continuously unwound and fed from a yarn supply package of a roll form by means of tensions in the warp yarns, viz., without aid of any positive drive means. The yarn supply package of the roll form is carried on a rotary structure which is usually called the yarn beam. The yarn beam has a shaft which is journaled on a stationary frame of the loom so as to be rotated about the shaft as the warp yarns carried thereon are unwound by a drawing effort which is imparted to the yarns from a weaving mechanism (usually including healds and reed as is well known in the art) of the loom. To provide positive control over the tensions in the yarns thus fed from the yarn beam, some looms are equipped with frictional braking units which are operatively associated with the yarn beams through the shafts of the yarn beams or through shafts which are driven for rotation from the shafts of the yarn beams. Typical of the braking units of this nature is such that the braking unit is selectively coupled to and uncoupled from the shaft of the yarn beam or the shaft rotatable with the shaft for the yarn beam in response to increases in the tensions in the warp yarns during weaving operation. Such selective actuation of the braking unit frequently invites sticking of the brake shoes or band to the brake disc or drum when they engage each other and it has therefore been difficult to achieve a prescribed frictional force between the braking elements, failing to provide a proper braking torque on the yarn beam and accordingly to enable the yarn beam to follow the variation in the tensions in the warp yarns.

A major cause of the variation in the tensions in the warp yarns fed from the yarn beam is the variation in an outside diameter of the roll of the yarns wound on the yarn beam although the tensions in the yarns vary due to various operating conditions of the loom during the weaving operation. As the warp yarns are unwound from the yarn beam, the outside diameter of the yarn supply package in the roll form on the yarn beam decreases from one layer of the yarns to another so that the length of the yarns unwound from the yarn beam per turn of the beam decreases to cause the tensions in the yarns to stepwise increase as the weaving operation proceeds. Thus, even though the braking torque imparted to the yarn beam may be successfully maintained constant through improvement of the braking elements to be used, control means should be provided so as to compensate for the increases in the tensions in the warp yarns preferably in close relation to increments of the tensions. Braking arrangements have been proposed and put into practice to achieve this end, using springs and/or weights the actions of which are controlled through sensing of the changes in the outside diameters of the yarn supply packages on the yarn beams. The changes in the outside diameters of the yarn supply packages will be to some extent representative of the changes in the tensions of the yarns being fed from the yarn beams when the looms are operating under normal conditions. When, however, the weaving cycles are varied as at the start or stop of the looms,

such variations can not be followed by the braking arnot be properly adjusted when the weaving cycles of the looms increase or decrease temporarily. in the braking arrangements using the springs and/or weights, furthermore, the braking torques imparted to the yarn beams are subject to undue variations resulting from changes in ambient temperatures or any other weather conditions and to the deposits of duct and moisture on the working surfaces of the brake discs or drums, providing another important cause which is responsible for the inability of properly controlling the tensions in the warp yarns. All these problems have been reflected by deterioration of the quality of the cloth produced in the looms using the warp yarn supply means of the negative let-off motion type. The present invention contemplates provision of a successful solution to these prob lems in a simple and economical construction.

It is, thus, an important object of the present invention to provide an improved yarn tension control mechanism for use in a loom having a warp yarn supply means of the negative let-off motion type.

it is another important object of the invention to provide an improved warp tension control mechanism which is adapted to achieve optimum tensions in the warp yarns fed from a warp yarn supply means of the negative let-off motion type.

It is still another important object of the invention to providean improved warp tension control mechanism incorporating a braking unit which is capable of applying an optimum braking torque on a warp yarn supply means of the netative let-off motion type throughout weaving operation of the loom.

It is still another important object of the present invention to provide an improved warp tension control mechanism having a brake unit which is controlled in relation to the tensions in the warp yarns being fed from the yarn supply means of the negative let-off motion type so that the yarn supply means is continuously braked to provide optimum tensions in the warp yarns in spite of the stepwise decrease in an outside diameter of the roll of the yarns on the supply means.

It is still another important object of the present invention to provide an improved warp tension control mechanism which is adapted to substantially continuously control to proper levels the tensions in the warp yarns from the yarn supply means in response to change in the outside diameter of the roll of the yarns on the supply means and to any unforseen condition resulting in change in the tensions in the warp yarns during weaving operation of the loom.

It is still another important object of the invention to provide an improved warp tension control mechanism including a brake control unit which is sensitive to any slight changes in the tensions of the warp yarns being fed from the yarn supply means of the negative let-off motion type during varying operational conditions of the loom.

in accordance with the present invention, these objects will be accomplished in a warp tension control mechanism which comprises in combination a brake wheel having a circumferential working surface and rotatable with the yarn supply means which'is usually in the form of a yarn beam, the brake wheel being driven for rotation about its axis by the yarn supply means when the yarn is fed from the supply means, a brake frame positioned around part of the circumferential working surface of the brake wheel, a first brake arm which is fast on one end portion of the brake frame, a second brake arm which is pivotally connected to the other end portion-of the brake and which extends substantially in parallel to and at a spacing from the first brake arm, the spacing between the first and second brake arms being variable by means of the pivotal connection between the brake frame and the second brake arm, at least two brake shoes one of which is carried on the brake frame and the other of which is carried by the second brake arm, biasing means for mechanically biasing the second brake arm toward the first brake arm for urging the brake shoes against the working surface of the brake wheel, the brake frame being urged to rotate about the axis of rotation of the brake wheel when a rotational force is imparted from the brake wheel to the brake frame through the brake shoes, and restraint means for restricting rotation of the brake frame rela- In one preferred embodiment of the present invention, the warp tension control mechanism may comprise, in combination, a brake wheel having a circumferential working surface and rotatable with the warp yarn supply means, the brake wheel being driven for rotation about its axis by the warp yarn supply means, a brake frame which is positioned around part of the circumferential working surface of the brake wheel, a first brake arm which is fast on one end portion of the brake frame, a second brake arm which is pivotally connected to the other end portion of the brake frame and which extends substantially in parallel to and at a spacing from the first brake arm, the spacing between the first and second brake arms being variable through the pivotal connection between the brake frame and the second brake arm, at least two brake shoes one of which is carried by the brake frame and the other of which is carried by the second brake arm, biasing means for mechanically biasing the second brake arm toward the first brake arm for urging the brake shoes against the working surface of the brake wheel, the brake frame being urged to rotate about the axis of rotation of the brake wheel when a rotational force is imparted from the brake wheel to the brake. frame through the brake shoes, restraint means for restricting the rotation of the brake frame relative to the brake wheel, the restraint means being associated with the biasing means for urging the brake frame to a rest position permitting the brake wheel to rotate relative to the brake frame against a braking torque applied "to the brake wheel from the brake shoes when the brake wheel is driven for rotation from the warp yarn supply means, and brake torque varying means responsive to the tension in the warp yarn being unwound and fed from the yarn supply means and associated with the restraint means for decreasing the biasing force imparted from the biasing means to the second brake arm and accordingly reducing the braking torque applied to the brake wheel from the brake shoes in response to an increase in the. tension in the warp yarn from the yarn supplymeans.

In another preferred embodiment of the present invention, the warp tension control mechanism may comprise in combination a brake wheel which has a circumferential working surface and which is rotatable with the warp yarn supply means, the brake wheel being driven for rotation aboutits axisby the warp yarn supply means when the yarn supply means is rotated by the yarn being unwound therefrom, a brake wheel positioned around part of the circumferential working surface of the brake wheel, a first brake arm which is fast on one end portion of the brake frame, a second brake arm which is pivotally connected to the other end portion of the brake frame and which extends substantially in parallel to and at a spacing from the first brake arm, the spacing between the first andsecond brake arms being variable through the pivotal connection between the brake frame and the second brake arm, at least two brake shoes one of which is carried'by the brake frame 'and theother of which is carried by the second brake arm, first biasing means for applying a first biasing force to the second brake arm for urging the second brake arm toward the first brake arm so that'the brake shoes on the brake frame and second brake arm are urged against the working surface of the brake wheel, the brake frame being urged to rotate about its axis when a rotational force is imparted from the brake wheel to the brake frame through the brake shoes, restraint means for restraining the brake frame from rotating around the brake wheel while permitting the brake wheel to rotate about its axis against a braking torque which is applied to the brake wheel from the brake shoes when the brake wheel is driven from the yarn supply means, and second biasing means responsive to the tension in the warpyarn being unwound and fed from the yarn supply means for applying a second biasing force to the second brake arm to urge the second brakearm away from the first brake arm so that the first biasing force and accordingly the braking torque applied to the brake wheel from the brake shoes are reduced in response to an increase in the tension in the warp yarn being'fed from the warp yarn supply means. I

The features and advantages of the warp tension control mechanism according to the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings, in which: I

FIG. 1 is a schematic side elevational view showing a preferred embodimentof the warp tension control mechanism according to the present invention;

FIG. 2 is a side elevational view showing a clutch and gear arrangement forming part of the brake torque varying means of the tension control mechanism depicted in FIG. 1; and

FIG. 3 is also a schematic side elevational view showing another preferred embodiment of the warp tension control mechanism according to the present invention.

Reference will now be made to the drawings, first to FIG. 1. As previously noted, the tension control mechanism herein disclosed in intended for use in a loom having a warp yarn supply means of the negative let-off motion type in which warp yarns are unwound and fed from the supply means by the tensions in the warp yarns and without aid of any positive or external drive means. The warp yarn supply means such a character is usually in the form of a yarn beam 10 having flanges 12 at both ends. The yarn beam 10 has a shaft 14 extending from both the flanges'l2 and journaled on a stationary frame structure (not shown) of the loom. The yarn beam 10 is thus adapted to carry thereon a warp yarn supply package 16 of yarns 18 which are wound on the beam in a roll form as indicated by a broken line. Substantially above the yarn beam 10 is positioned a guide roller which is rotatably supported on a shaft 20a journaled on the stationary frame structure of the loom. A back-rest roller 22 is positioned posterior to this guide roller 20 and is rotatably supported on a shaft 22a. The shaft 22a of the back-rest roller 22 is fixedly connected to the shaft 20a of the guide roller 20 through a connecting member 24 so that the back-rest roller 22 is rotatable not only about the shaft 22a but the shaft 20a of the guide roller 20 through the connecting member 22. The warp yarns 18 from the yarn beam 10 are passed through these guide roller 20 and back-rest roller 22 over to healds 26 so as to be shed to received a weft yarn as customary. The warp yarns 18 are drawn by means (not shown) which are positioned posterior to the healds 26 so that the yarns 18 are unwound and fed from the yarn beam 10 by the tensions in the warp yarns being passed through the guide roller 20 and back-rest roller 22 without application of any positive driving effort to the yarn beam 10 during weaving operation. The present invention is directed to the mechanism for controlling the tensions in the warp yarns 18 being fed from the yarn beam 10 and, as such, no further detailed description will be herein incorporated as to the construction of the weaving and drawing mechanism forming part of the loom.

The warp tension control mechanism for use in the yarn beam 10 of the negative let-off motion type in accordance with the present invention consists essentially of a brake unit which is adapted to apply a braking force to the yarn beam in response to a rotational force imparted thereto from the yarn beam 10 and a brake control unit which is adapted to control the braking torque of the brake unit in accordance with the tensions in the warp yarns.

The brake unit comprises a drive gear 28 which is securely carried on the shaft 14 of the yarn beam 10 and a driven gear 30 which is in constant mesh with the drive gear 28 and which is supported on a shaft 32 journaled on the stationary frame structure (not shown) of the loom. The shaft 32 further carries thereon a brake disc or wheel 34 which is thus rotatable with the driven gear 30 and which has a circumferential working surface 34a. An approximately semi-circularbrake frame 36 is positioned around part of the circumferential working surface 34a of the brake disc or wheel 34. The brake frame 36 is integral at one end thereof with a first brake arm 38 and is pivotally connected at the other end thereof to a second brake arm 38' through a pivotal pin 40. The first and second brake arms 38 and 38, respectively, extend substantially in parallel to and at a spacing from each other so that the brake frame 36 and the brake arms 38 and 38' are arranged in a generally U-shaped configuration. The spacing between the brake arms 38 and 38' is variable by reason of the piv otal connection between the brake frame 36 and the second brake arm 38'. The brake frame 36 carries at its inner peripheral end two brake shoes 42a and 42b while the second brake arm 38' carries at its inner end a brake shoe 42c. The brake shoes 42a, 42b and 420 are substantially equiangularly spaced apart from each other around the axis of the brake wheel 34, viz., spaced apart 120 from each other and are adapted to engages at their inner faces the working surface 34a of the brake wheel 34.

The first brake arm 38 is provided with an extension 44 which is directed substantially in parallel to the spacing between the brake arms 38 and 38', viz., ap' proximately transverse to the arms. The extension 44 terminates approximately halfway of the spacing between the brake arms 38 and 38' so as to be substantially aligned with the axis of rotation of the brake wheel 34 as will be seen in FIG. 1. The extension 44 pivotally carries at its leading end a guide lever 46 through a pivotal pin 48. The guide lever 46 has an end portion appreciably extending beyond the extension 44 toward the brake wheel 34 and extends away from the brake wheel. A first tension spring 50 is anchored at one end to the end portion of the guide member 46 close to the brake wheel 34 and connected at the other end to the first brake arm 38 through a fitting 52 which is fastened to the brake arm 38 by an adjustable bolt 54 and nut 56. Likewise, a second tension spring 50' is anchored at one end to the end portion of the guide member 46 and connected at the other to the second brake arm 38' through a fitting 52 which is fastened to the brake arm 38' by an adjustable bolt 54' and nut 56. The first and second tension springs 50 and 50', respectively, usually have a common spring contant so that the second brake arm 38 is urged toward the first brake arm 38, thereby holding the guide lever 46 substantially perpendicular to the extension 44. With the second brake arm 38' thus urged to turn toward the first brake arm 38 about the pivotal pin 40, the brake shoe 42c on the second brake arm 38 is forced against the working surface 34a of the brake wheel 34 with the result that the other brake shoes 42a and 421) are forced against the working surface 34a of the brake wheel 34. The brake wheel 34 is thus pressed upon by the three equiangularly spaced brake shoes 42a, 42b and 42c in such a manner that the vector forces exerted from the brake shoes become zero when combined. The brake frame 36, brake arms 38 and 38' and extension 44 are, therefore, supported on the brake wheel solely by means of the pressing engagement of the brake shoes 42a, 42b and 420 with the working surface 34a of the brake wheel 34 and are not subject to restraint by any other mechanical means.

Although, in the above description, the brake shoes have been assumed to be three in number, such is merly by way of example and, therefore, only two or even more than three brake shoes may be used where desired. It is, in this instance, preferable to have the brake shoes located substantially equiangularly from each other so as to enable them to impart a uniform braking torque to the brake wheel. However, the intent of the present invention will be maintained even in an arrangement in which the brake shoes carried on the brake frame are located in symmetry with respect to a line joining the center of the brake shoe carried on the second brake arm and the axis of rotation of the brake wheel.

The guide lever 46 is in constant engagement at its lengthwise edge with a stop or fulcrum member 58 which is herein assumed to be held stationary providing restraint on the angular movement of the guide lever 46 about the pivotal pin 48. As will be described later, however, the stop or fulcrum member 58 is movable in a direction substantially parallel to the guide lever 46 assuming a neutral position which is shown in FIG. 1. This direction will be referred to as a vertical direction for the simplicity of description although the brake unit of the construction thus far described may assume'any relative position other than that illustrated in FIG. 1.

Similarly, the terms upward and downward and clockwise and counter clockwise as will be used hereinafter should be interpreted merely as indicating the directionsof motions relative to the drawings and not as limitative of the actual or practically possible directions-of motions in the mechanism implementing the present invention.

When, now,.the loom is inoperative with the yarn beam 10 held'at rest, the tension springs 50 and 50 coact with each other for maintaining the guide lever 46 in the neutral position illustrated and forcing the brake shoes 42a, 42b and 42c tightly against the working surface 34a of the brake wheel 34 with equal braking forces. The braking forces thus exerted on the brake wheel 34 can be adjusted by varying the spring constants of the tension springs 50 and 50 through adjustment of the bolts 54 and 54' and nuts 56 and 56', respectively. The guide lever 46 thus held in the neutral position extends in a vertical direction which is substantially in alignment with the center of the brake wheel 34 so that an angle of the guide lever 46 from the extension 44 (which is assumed to extend substantially horizontally in .the drawing) is approximately 90.

When, then, the loom starts the weaving operation, the warp yarns 18 are drawn toward the weaving station including the healds 26 through the guide roller 20 and back-rest roller 22. The warp yarns 18 are thus continuously unwound and fed in a row from the yarn supply package 16 on the yarn beam by means of the tensions produced therein by the drawing force imparted to the yarns from the weaving station. The yarn beam 10 is accordingly rotated about the shaft 14 in clockwise direction as indicated by an arrow A with a circumferential velocity which is substantially equal to the velocity at which the yarns 18 advance through the guide roller 20 and back-rest roller 22. This causes the drive gear 28 to rotate about the shaft 14 for causing, in turn, the driven gear 30 to rotate about the shaft 32 so that the brake wheel 34 carried on the shaft 32 is driven to rotateabout its axis in counter clockwise direction as indicated by an arrow B. Under this condition, the brake shoes 42a, 42b and 420 are tightly forced against the working surface 34a of the brake wheel 34 by the'actions of the tension springs 50 and 50" so that the brake frame 36 together with the brake arms 38 and 38' and extension 44 is tugged by the brake wheel 34 rotating about the shaft 32 and is accordingly rotated in the counter clockwise direction through a certain angle. The rotation of the brake frame 36 about the shaft 32 is followed by pivotal movement in clockwise direction of the guide lever 46 about the pivotal pin 48 on the extension 44 and about the fulcrum member 58 which is in engagement with the lengthwise edge of the guide lever 46. As a consequence, the first tension spring 50 is elongated from the initial length providing an increased spring action while the second tension spring 50 is compressed from the initial length providing a reduced spring action. The second brake arm 38' loaded by the second tension spring 50' is pivotally connected to thebrake frame 36 as previously described so that the decrease in the spring action of the tension spring 50' is followed by reduction in the compressive force of the brake shoe 420 on the working surface 34a of the brake wheel 34 and accordingly by decreases in the braking forces of the other brake shoes 42a and 42b on the brake wheel 34 by reason of the action and reaction. The brake wheel 34 thus overcomes the braking torque applied thereto from the brake shoes 42a, 42b and 42c and is allowed to rotate about the shaft 32 in unison with the yarn beam 10, maintaining the brake frame 36 in a rest position by means of the restraint from the guide member 46 and by the increased spring action of the first tension spring 50. The frictional forces exerted between the working surface 34a of the brake wheel 34 and the individual brake shoes 42a, 42b and 420 can thus be maintained substantially constant because the braking action of each of the brake shoes decreases as the brake frame 36 is urged to turn together with the brake wheel 34. The yarn beam 10 is therefore, subject to a substantially constant braking torque insofar as the yarn beam 10 is driven by the warp yarns 18 with a fixed torque.

As will be understood from the foregoing description, the braking torque imparted to the yarn beam 10 varies depending upon the angle 0 of the guide lever 46 to the extension 44, viz., upon the relative angular positions of the guide lever 46 and the brake frame 36. If, forexample, the angle 0 decreases as in the condition in which the brake frame36 is tugged by the brake wheel 34 and brought into the rest position as describedabove, the tension springs 50 and 50' exert differential spring actions so that the guide lever 46 is urged in counter clockwise direction and accordingly the brake frame 36 urged in clockwise direction which is opposite to the direction of arrow B. If, therefore, the yarn beam 10 is driven by the yarns 18 with a constant torque, then the brake frame 36 will be held in the rest position with the angle 6 maintained substantially constant so that the tensions in the yarns 18 being fed from the yarn beam 10 will be also maintained constant. The fact is, however, that the yarn beam 10 is driven by the yarns 18 with a stepwise increasing torque because the outside diameter of the roll 16 of the yarns on the yarn beam 10 decreases from one layer of the yarns to another as the weaving operation proceeds and, for this reason, it is necessary for maintaining constant the tensions in the warp yarns 18 to have the angle 0 reduced as the diameter of the roll 16 of the yarns decreases. This purpose will be accomplished in the warp tension control mechanism according to the present invention through provision. of a brake control unit which is combined with the brake unit of the construction previously described.

When the brake frame 36 is turned a certain central angle around the brake wheel 34 as a result of the rotation of the brake wheel about the axis thereof, the guide lever 46 will be moved in unison with the extension 44 with the consequent decrease in the angle 0 between the guide lever 46 and the extension 44 as already explained. The decrement of the angle 0 is dictated by a distance between the pivotal pin 48 and the engaging point of the guide lever 46 with the fulcrum member 58 which has been assumed as being held stationary for conveniences sake. Thus, the greater the distance between the pivotal pin 48 and the fulcrum member 58, the smaller the angle 0 and accordingly the smaller the braking turque imparted from the brake shoes 42a, 42b and 42c to the brake wheel 34, and vice versa. The brake control unit forming part of the warp tension control mechanism according to the present invention is adapted to move the fulcrum member 58 away from the pivotal pin 48 as the weaving operation proceeds and accordingly the outside diameter of the roll 16 of the yarns on the yarn beam decreases.

For this purpose, the fulcrum member 58 is carried on a slider 60 which is slidably mounted on a guide rail 62 extending in a vertical direction which is substantially parallel to the direction in which the guide lever 46 extends when it is held in the neutral position as previously mentioned. The guide rail 62 is secured in position on the stationary frame structure of the loom through brackets 64 and 64' which are connected to both ends of the guide rail. The slider 60 has a portion 60a which is internally screw-threaded and engages through this internally screw-threaded portion 60a with an externally screw-threaded guide rod 66 which extends along the entire length of the guide rail 62 and which is journaled at both ends on the brackets 64 and 64'. The guide rod 66 is rotatable about its axis so that the slider 60 carried thereon is driven to move axially of the guide rod by means of the engagement between the screw-threaded portion 60a of the slider 60 and the guide rod. To facilitate the sliding movement of the slider 60 on the guide rail 62, the slider 60 may be provided with roller 68 which is in rolling contact with the guide rain 62. Designated by reference numeral 68 is a handle for permitting manual adjustment of the realtive position of the slider 60 on the guide rod 66.

The guide rod 66 has an axial extension 66a which terminates in close proximity to the driven gear 30 on the shaft 32 for carrying thereon a clutch and gear arrangement which is adapted to be driven substantially in synchronism with the brake wheel 34 and accordingly with the yarn beam 10. The clutch and gear arrangment, indicated by broken lines and designated as a whole by reference 70 in FIG. 1, is illustrated in detail in FIG. 2.

Referring to FIG. 2, the extension 66a carries at its leading end portion of a spiral gear 72 which is rotatable on the extension 66a. The spiral gear 72 is in constant mesh with a spiral gear 74 which is fixedly carried on the shaft 32 supporting the brake wheel 34 so that the rotation of the shaft 32 is carried over to the spiral gear 72 through the spiral gear 74. The spiral gear 72 is integral on one face thereof with a clutch half 76 which is engagable with a clutch half 78. The clutch half 76 is rotatable on the extension 66a of the guide rod 66 similarly to the spiral gear and is prevented by suitable means for moving axially of the extension 66a. The clutch half 78, on the other hand, is fixedly mounted on and accordingly rotatable with the extension 66a of the guide rod 66 and is keyed or splined or otherwise axially movably carried on the extension 66a. The clutch half 78 is thus adapted to be coupled to and uncoupled from the clutch half 76 as it is moved axially of the extension 66a of the guide rod 66. The extension 66a of the guide rod 66 is rotatably received at its end in a mount 79 which is fast on a stationary frame structure (not shown) of the loom.

The clutch half 78 secured on the extension 66a of the guide rod 66 is formed with an annular groove 78a for receiving therein an appropriate member which is adapted to drive the clutch half 78 to move axially of the extension 66a of the guide rod 66 responsive to variation in the tensions in the warp yarns being fed from the yarn beam (FIG. 1).

In accordance with the present invention, the reciprocating movement of the clutch 78 is brought about by means of a solenoid-operated rotary motion device 80. The solenoid-operated rotary motion device 80 has a plunger 80a which is rotatable about its axis as the solenoid incooporated in the rotary motion device 80 is energized in a manner to be described. The plunger 80a is in constant engagement with the clutch half 78 through the annular groove 78a so as to move the clutch half 78 to a position coupled with the clutch half 76 when the plunger 80a is turned in response to actuation of the rotary motion device 80.

An arrangement for actuating the solenoid-operated rotary motion device is illustrated in FIG. 1. Turning back to FIG. 1, the solenoid in the rotary motion device 80 has a coil (not shown) which are connected by lines 82 to a limit switch 84 through a power source 86. To actuate this limit switch 84 in response to increases in the tensions in the warp yarns 18 being fed from the yarn beam 10, a lever 88 is secured to the shaft 20a carrying the guide roller 20. The lever 88 is angularly spaced apart from the connecting member 24 interconnecting the shafts 20a and 22a of the guide and back rest rollers 20 and 22, respectively, and is thus pivotally movable about the shaft 20a as the back-rest roller 22 is turned about the shaft 20a. The lever 88 is in engagement at its leading end with a roller 90 which is rotatably carried on a rocking lever 92. The rocking lever 92 is pivotally mounted on a shaft 94 and has approximately diametrically opposed arms 92a and 92b, the roller 90 being carried on the arm 92a. The other arm 92b has its end portion in close proximity to and accordingly engageable with an actuating element 84a of the limit switch 84. With the levers 88 and 92 arranged in the relative positions illustrated in FIG. 1, the lever 88 is turned clockwise as the back-rest roller 22 turns clockwise and such clockwise rotation of the lever 88 causes the rocking lever 92 to turn counter clockwise about the shaft 94 through the roller 90. The counter clockwise rotation of the rocking lever 92, in turn, causes tha actuating element 84a of the limit switch 84 to be depressed by the leading end of the arm 92b so that the limit switch 84 closes to energize the solenoid coil in the solenoid-operated rotary motion device 80. To normally maintain the limit switch 84, the rocking lever 92 is urged by suitable mechanical biasing means to turn clockwise about the shaft 94 so that the arm 92b thereof is biased to be disengaged from the actuating element 84a of the limit switch 84. The mechanical biasing means is herein assumed to be a coiled tension spring 96 which is anchored at one end to the arm 92!; and at the other end to a pin 98 which is fast on a stationary frame structure (not shown) of the loom.

As previously described, the brake unit of the warp tension control mechanism according to the present invention is adapted to produce a substantially constant braking torque without respect of the variation in the tensions in the warp yarns fed from the yarn beam 10. The brake control unit having the construction above described is intended to control the brake unit for regulating the braking torque in response to the variation in the warp yarns from the yarn beam throughout the weaving operation of the loom.

When, thus, the tensions in the warp yarns 18 being fed from the yarn beam 10 to the healds 26 through the guide roller 20 and back-rest roller 22, then the backrest roller 22 will be urged in the forward direction of the warp yarns 18 which are being passed therethrough. The back-rest roller 22 is consequently forced to turn clockwise about the shaft 20a carrying the guide roller 20 so that the lever 88 is also turned clockwise of the shaft 20a through the connecting member 24. The lever 88 then depresses the roller 90 on the arm 92a of the rocking lever 92 which is consequently turned about the shaft 94 against the action of the tension spring 96, thereby raising the arm 92b until the arm 92b abuts against and presses upon the actuating element 84a of the limit switch 84. The limit switch 84 now closes so as to energize the coil in the solenoidoperated rotary motion device 80. The plunger 82a of the rotary motion device 80 then commences to turn about its axis so that the clutch half 78 engaged thereby is driven to move into engagement with the clutch half 76. The spiral gear 72 is constantly driven to rotate on the extension 66a of the guide rod 66 from the shaft 32 through the .spiral gear 74 so that, when the clutch halves 76 and 78 are coupled with each other, the rotational force from the shaft 32 is carried over to the guide rod 66 through the spiral gears 74 and 72, clutch halves 76 and 78 and extension 66a of the guide rod 66. The shaft 32 being rotated in unison with the yarn beam 10 through the gears 28 and 30 as previously discussed, the guide rod 66 is rotated about its axis as the yarn beam 10 is driven to rotate about the shaft 14 by means of the tensions in the warp yarns 18 being unwound therefrom. When the guide rod 66 is driven to rotate in this manner, the slider 60 is moved upwardly, viz., away from the extension 66a through engagement between the internally screw-threaded portion 60a of the slider 60 and the externally screw-threaded guide rod 66. This upward movement of the slider 60 is guided by the guide rail 62 and the rollers 68 which roll on the guide rail.

The stop or fulcrum member 58 which is fast on the slider 60 is in this manner moved away. from the brake wheel 34 as the outside diameter of the roll 16 of the yarns on the yarn beam 10 decreases and accordingly the tensions in the yarns 18 passing through the backrest roller 22 increase. This will result in reduction of the angle between the guide lever 46 and the extension 44 from the first brake arm 38 on the brake frame 36 and consequently in continuous reduction of the braking torque applied from the brake shoes 42a, 42b and 420 to the brake wheel 34. The braking action of the yarn beam is thus reduced so that the tensions in the yarns 18 being fed from the yarn beam decrease to a proper level. When the proper level is reached by the tensions in the yarns, then the back-rest roller 22 will restore its initial position so that the rocking lever 92 is allowed to turn counter clockwise about the shaft 94 by the action of the tension spring 96. The limit switch 84 then opens and accordingly the solenoidoperated rotary motion device 80 is de-energized to permit the plunger 80a to turn to its initial position for moving the clutch half 78 out of engagement with the clutch half 76. the clutch half 78 and accordingly the guide rod 66 thus cease to rotate while the clutch half 76 integral with the spiral gear 72 continues to rotate driven constantly by the spiral gear 74. The clutch halves 76 and 78 are in this manner coupled and uncoupled repeatedly as the tensions in the warp yarns 18 passing through the back-rest roller 22 are about to increase beyond the proper level whereby the tensions of the yarns are maintained substantially at a constant optimum value which is dictated by the geometry of the levers 88 and 92 and the spring constant of the tension spring 96.

The slider will be moved to the upper end of the guide rod 66 remote from the brake wheel 34 at a final stage of the weaving operation when the yarn beam 10 is empty. For the succeeding weaving operation with the yarn beam 10 replaced with a full one, the slider 60 should be driven to move to the lower end of the guide rod 66 close to the brake wheel 34 by rotating the guide rod at the handle 69 in opposite direction.

FIG. 3 illustrates another preferred embodiment of the warp tension control mechanism according to the present invention. The tension control mechanism herein shown is also made up of a brake unit adapted to produce a substantially constant braking torque and a brake control unit which is adapted to control the brake unit for substantially continuously adjusting the braking torque so as to maintain the tensions in the warp yarns from the yarn beam to a proper level throughout-the varying operational conditions of the loom. The mechanism shown in FIG. 3 features an increased sensitivity to the variation of the tensions in the yarns resulting from the stepwise decrease of the outside diameter of the yarn supply package in a roll form of the yarns on the yarn beam and from unforeseen causes which may be invited in the weaving station of the loom.

Referring to FIG. 3, the yarn beam, yarns fed from the yarn beam, guide and back rest rollers positioned in the paths of the warp yarns, and healds forming part of the weaving station are designated by reference numerals 10, 18, 20, 22 and 26, respectively, and the yarn beam 10, guide roller 20 and back-rest 20 have shafts 14, 20a and 22a, respectively, similarly to the mechanism illustrated in FIG. 1. The shafts l4 and 22a of the yarn beam 10 and guide roller 20 are journaled on stationary frame structure of the loom while the shaft 22a of the back-rest roller 22 is fixedly connected to the shaft 20a of the guide roller 20 through the connecting member 24.

While the brake wheel 34 are driven for rotation from the yarn beam 10 through the gears 28 and 30 and the shaft 32 carrying the gear 30 and brake wheel 34 in the mechanism shown in FIG. 1, the warp tension control mechanism illustrated in FIG. 3 has a brake wheel 100 which is directly carried on the shaft 14 so as to be rotatable with the yarn beam 10 as'the yarns 18 are unwound and fed from the yarn beam by means of the tensions in the yarns. Around part of the circumferential working surface of the brake wheel 100 is positioned a substantially semi-circular brake frame 102. The brake frame 102 is integral at one end portion there of a first brake arm 104 extending substantially outwardly radially of the brake frame and is pivotally connected at the other end portion thereof to a second brake arm 104' through a pivotal pin I06. The second brake arm 104' extends approximately in parallel to and at a spacing from the first brake arm 104, the arms 104 and 104 being shown as extending substantially horizontally in FIG. 3. The brake frame 102 has formed on its outer peripheral edge spaced parallel projections second brake arm 104 carries on its inner edge a third brake shoe 112c. The brake shoes 112a, 112b and 1120 are in contact with the brake wheel 100 and are preferably substantially equiangularly spaced apart from each other around the center of the brake wheel 100 as seen in FIG. 3. As previously discussed in connection with the brake shoes incorporated in the tension control mechanism illustrated in FIG. 1, the number of the brake shoes may be increased or decreased as desired and it is permissible that the brake shoes are not in symmetrical with each other with respect to the center of the brake wheel 100 insofar as the brake shoes on the brake frame 102 are in symmetry with each other with respect to the line joining the center of the brake shoe on the second brake arm 104 and the center of the brake wheel 100.

Suitable mechanical biasing means is provided so as to urge the second brake arm 104 toward the first brake arm 104. The biasing means is herein exemplified as a coiled tension spring 114 which is anchored at one end to the second brake arm 104 through a pin 116 fixed on the arm 104 and connected to the other end to the first brake arm 104 through an adjustable bolt 120 which is adjustably fastened to the brake arm 104 by means of a nut 122. The second brake arm 104' is consequently urged toward the second brake arm 104so that the third brake shoe 112c is forced against the working surface 100a of the brake wheel 100 whereby the other brake shoes 112a and l12b are also pressed upon the working surface of the brake wheel 100 by reason of Newtons third law of motion.

While the mechanical biasing means operative to urge the second brake arm 104' to turn counter clockwise of the drawing has been assumed to be the coiled tension spring 114 in the above discussion, such is merely by way of example so that a weight and pulley arrangement may be utilized where desired. In this instance, a rope or any other flexible line may be anchored at one end to the first brake arm 104, passed on a pulley mounted on the second brake arm 104' and connected to the other end to a weight which is thus suspended from the pulley on the second brake arm 104, though not shown in FIG. 3.

The mechanical biasing means, which may thus be in any desired form, forms essential part of the brake unit of the tension control mechanism shown in FIG. 3, causing the brake shoes 112a, 112b and 112C to apply a substantially uniform and constant braking torque to the brake wheel 100 as the brake wheel is driven by the yarn beam to rotate about the shaft 14 essentially similarly to the brake unit of the mechanism shown in FIG. 1.

The braking torque thus exerted by the described brake unit is controlled by the brake control unit which is responsive to the variation in the tensions in the warp yarns 18 fed from the yarn beam 10.

To sense the tensions in the warp yarns 18 being fed from the yarn beam 10 to the healds 26, the brake control unit comprises a bell-crank lever 124 which is fast on and rotatable with the shaft 20a of the guide roller 20. The bell-crank lever 124 has angularly spaced arms 124a and l24b which are rotatable about the shaft 20a as the connecting member 24 interconnecting the shafts 20a and 22a of the guide and back-rest rollers 20 and 22. One arm 124a of the bell-crank lever 124 carries at its free end a pin 126. Suitable mechanical biasing means such as a coiled tension spring 128 is connected at one end to this pin 126 through a substantially straight rod portion 128a and at the other end to the pin 116 on the second brake arm 104' of the brake unit. This tension spring 128 is adapted to provide a varying load on the second brake arm 104 opposing the action of the tension spring 114, and may be replaced with a weight and pulley arrangement which was discussed previously with regard to the tension spring 114. Or otherwise desired, the arm 1240: may be connected to the second brake arm 104' by a substantially rigid member if the second brake arm 104' is formed with a pliable material such as a spring steel.

When, now, the outside diameter of the roll (not seen in FIG. 3) of the yarns on the yarn beam 10 stepwise decrease from one layer of the yarns to another as the weaving operation proceeds in the loom, then the tensions in the warp yarns 18 being passed from the backrest roller 22 tend to increase. The back-rest roller 22 is therefore caused to turn clockwise of the drawing about the shaft 20a of the guide roller 20 through the connecting member 24, causing the bell-crank lever 124 to accordingly turn clockwise about the shaft 20a. The arm 124a of the bell-crank lever 124 is consequently turned upwardly so that the rod portion 128a of the tension spring 128 is raised from its initial position, viz., away from the second brake arm 104' with a consequent increase in the spring force of the tension spring 128. This causes the second brake arm 104 to be urged toward the first brake arm 104 with a lesser force due to an increased opposing effort applied from the tension spring 128 with the result that the compressive forces exerted on the working surface of the brake wheel decrease. The brake wheel 100 is accordingly subjected to a braking torque which is so lessened as to be appropriate for rendering the tensions in the warp yarns 18 to a proper level. The spring action of the tension spring 128 is thus substantially continuously increased as the outside diameter of the roll of the yarns on the yarn beam 10 decreases so that the tensions in the warp yarns 18 fed from the yarn beam are at all times maintained at the proper level.

The arm 12412 of the bell-crank lever 124 and parts associated therewith are intended to apply tensions to the warp yarns l8 fed to the healds 26 at an incipient stage of the weaving operation and to substantially compensate for unexpected changes in the tensions in the warp yarns 18 as would be caused by the shedding and beating of the yarns at the weaving station of the loom during operation. Thus, the arm 124b is pivotally connected at its free end to a pivotal member 130 which is in turn connected through an adjustable bolt 132 and nut 134 to a spring seat member 136. A coiled tension spring 138 is anchored at one end to this spring seat member 136 and at the other end to a spring seat member which is secured to a stationary frame structure of the loom. The tension spring 138 thus urges the bell-crank lever 124 and accordingly the back-rest roller 22 to turn counter clockwise and may be replaced with mechanical biasing means of any form other than the coiled tension spring. The arrangement including the arm l24b of the bell-crank lever 124 and the tension spring 138 is provided merely by preference so that it may be dispensed with where desired.

What is claimed is:

1. A warp tension control mechanism for a loom having warp yarn supply means driven for rotation by a tension in a warp yarn fed from a yarn supply package of a roll form on said warp yarn supply means comprising, in combination, a brake wheel having a circumferential working susrface and rotatable with said warp yarn supply means, the brake wheel being driven to rotate about its axis by said yarn supply means, a brake frame which is positioned around part of said working surface of said brake wheel, a first brake arm which is fast on one end portion of said brake frame, a second brake arm which is pivotally connected to the other end portion of said brake frame and which extends substantially in parallel to and at a spacing from said first brake arm, said spacing between the first and second brake arms being variable by means of the pivotal connection between said brake frame and said second brake arm, at least three brake shoes one of which is carried on said second brake arm and the others of which are carried on said brake frame, the brake shoes on the brake frame being equidistantly spaced apart from the brake shoe on the second brake arm, said first brake arm having an extension having a leading end portion which is located substantially halfway of said spacing betweenthe first and second brake arms, a guide lever which is connected through a pivot to said leading end portion of said extension and which extends away from said brake wheel, first and second mechanical biasing means associated respectively with said first and second brake arms and urging said guide lever to a neutral position which is substantially in alignment with the axis of rotation of said brake wheel, a movable fulcrum member which is in sliding engagement with said guide lever for allowing the guide lever to turn about the fulcrum member and about the pivotal connection between the guide lever and said leading end portion of said extension, said brake frame being urged to turn about the axis of rotation of said brake wheel to a rest position with said first mechanical biasing means caused to have its biasing force increased and said second mechanical biasing means caused to have its biasing force reduced as a rotation force is imparted from said brake wheel to the brake frame through said brake shoes for moving said guide lever to a position which is angularly spaced apart from said extension at a reduced angle and for permitting the brake wheel to rotate about its axis by a rotational force imparted thereto from said yarn supply means, and brake torque varying means responsive to variation in the tension of the warp yarn being unwound and fed from said yarn supply means and associated with said fulcrum member for decreasing said angle between said guide lever and said extension in response to an increase in the tension in the yarn so that the biasing force of said second mechanical biasing means is reduced and said brake shoes are urged against said brake wheel with a reduced biasing force.

2. A warp tension control mechanism as claimed in claim 1, in which said brake torque varying means comprises a slider which carries thereon said fulcrum member, guide means for guiding said slider in a path which is substantially parallel to a direction in which said guide lever assumes when it is in said neutral position aligned with the axis of rotation of said brake wheel, and drive means responsive to variation in the tension in the warp yarn being fed from said yarn supply means and operative to be driven from the yarn supply means, said drive means being disconnected from said guide means when the tension in the warp yarn fed from the yarn supply means is maintained under a predetermined level and drivingly connected thereto in response to an increase in the tension in the yarn.

3. A warp tension control mechanism as claimed in claim 2, in which said guide means comprises an internally screw-threaded portion forming part of said slider and an externally screw-threaded guide rod engaging with said internally screw-threaded portion and extending along said path, said drive means being drivingly connected to said guide rod in response to the increase in the warp yarn from the yarn supply means for rotating the guide rod about its axis so that said internally screw-threaded portion of the slider is moved along the guide rod.

4. A warp tension control mechanism as claimed in claim 2, in which said drive means comprises clutch means having a first clutch half rotatable with said yarn supply means and constantly driven therefrom and a second clutch half engageable with said first clutch half and drivingly connected to said guide means, electromagnetic actuating means in mechanical engagement with said second clutch half and having an inoperative position holding the second clutch half uncoupled from said first clutch half and an inoperative position moving said second clutch half into engagement with said first clutch half for driving said guide means, electric switch means electrically connected to said actuating means,

a control lever which is mechanically engageable with said switch means and which is operative to be moved responsive to an increase in the tension in the warp yarn being fed from said yarn supply means for thereby actuating said switch means to close and energize said electromagnetic actuating means, and mechanical biasing means uring said control lever to a position disengaged from said switch means.

5. A warp tension control mechanism as claimed in claim 1, in which said brake torque varying means comprises a slider which carries thereon said fulcrum member, an internally screw-threaded portion forming part of said slider, an externally screw-threaded guide rod engaging said internally screw-threaded portion and engaging substantially in parallel to a direction in which said guide lever assumes when it is in said neutral position aligned with the axis of rotation of said brake wheel, clutch means having a first clutch half which is rotatable with and constantly driven from saidv yarn supply means and a second clutch half engageable with said clutch half and drivingly connected to said guide rod, said guide rod and said second clutch half being driven to rotate about their axes when the clutch halves are coupled together, electromagnetic actuating means in mechanical engagement with said second clutch half and having an operative position holding said second clutch half uncoupled from said first clutch half and an inoperative position moving the second clutch half into engagement with said first clutch half for driving said guide rod from said yarn supply means through said first clutch half, electric switch means electrically connected to said actuating means, a control lever which is mechanically engageable with said switch means and which is operative to be moved responsive to an increase in the tension in the warp yarn being fed from said yarn supply means for thereby actuating said switch means to close and energize said electromagnetic actuating means, and mechanical biasing means urging said control lever toward a position disengaged from said switch means.

claim 1, in which said brake shoes carried on said brake frames are located substantially in symmetry with re spect to a line joining a center of said brake shoe carried on said second brake arm and the axis of rotation of said brake wheel.

8. A warp tension control mechanism as claimed in claim 7, in which said at least three brake shoes are located substantially equiangularly around said axis of rotation of said brake wheel.

l= =l= l =l 

1. A warp tension control mechanism for a loom having warp yarn supply means driven for rotation by a tension in a warp yarn fed from a yarn supply package of a roll form on said warp yarn supply means comprising, in combination, a brake wheel having a circumferential working susrface and rotatable with said warp yarn supply means, the brake wheel being driven to rotate about its axis by said yarn supply means, a brake frame which is positioned around part of said working surface of said brake wheel, a first brake arm which is fast on one end portion of said brake frame, a second brake arm which is pivotally connected to the other end portion of Said brake frame and which extends substantially in parallel to and at a spacing from said first brake arm, said spacing between the first and second brake arms being variable by means of the pivotal connection between said brake frame and said second brake arm, at least three brake shoes one of which is carried on said second brake arm and the others of which are carried on said brake frame, the brake shoes on the brake frame being equidistantly spaced apart from the brake shoe on the second brake arm, said first brake arm having an extension having a leading end portion which is located substantially halfway of said spacing between the first and second brake arms, a guide lever which is connected through a pivot to said leading end portion of said extension and which extends away from said brake wheel, first and second mechanical biasing means associated respectively with said first and second brake arms and urging said guide lever to a neutral position which is substantially in alignment with the axis of rotation of said brake wheel, a movable fulcrum member which is in sliding engagement with said guide lever for allowing the guide lever to turn about the fulcrum member and about the pivotal connection between the guide lever and said leading end portion of said extension, said brake frame being urged to turn about the axis of rotation of said brake wheel to a rest position with said first mechanical biasing means caused to have its biasing force increased and said second mechanical biasing means caused to have its biasing force reduced as a rotation force is imparted from said brake wheel to the brake frame through said brake shoes for moving said guide lever to a position which is angularly spaced apart from said extension at a reduced angle and for permitting the brake wheel to rotate about its axis by a rotational force imparted thereto from said yarn supply means, and brake torque varying means responsive to variation in the tension of the warp yarn being unwound and fed from said yarn supply means and associated with said fulcrum member for decreasing said angle between said guide lever and said extension in response to an increase in the tension in the yarn so that the biasing force of said second mechanical biasing means is reduced and said brake shoes are urged against said brake wheel with a reduced biasing force.
 2. A warp tension control mechanism as claimed in claim 1, in which said brake torque varying means comprises a slider which carries thereon said fulcrum member, guide means for guiding said slider in a path which is substantially parallel to a direction in which said guide lever assumes when it is in said neutral position aligned with the axis of rotation of said brake wheel, and drive means responsive to variation in the tension in the warp yarn being fed from said yarn supply means and operative to be driven from the yarn supply means, said drive means being disconnected from said guide means when the tension in the warp yarn fed from the yarn supply means is maintained under a predetermined level and drivingly connected thereto in response to an increase in the tension in the yarn.
 3. A warp tension control mechanism as claimed in claim 2, in which said guide means comprises an internally screw-threaded portion forming part of said slider and an externally screw-threaded guide rod engaging with said internally screw-threaded portion and extending along said path, said drive means being drivingly connected to said guide rod in response to the increase in the warp yarn from the yarn supply means for rotating the guide rod about its axis so that said internally screw-threaded portion of the slider is moved along the guide rod.
 4. A warp tension control mechanism as claimed in claim 2, in which said drive means comprises clutch means having a first clutch half rotatable with said yarn supply means and constantly driven therefrom and a second clutch half engageable with said first clutch half and drivingly connected to said guide means, electromagnetic actuating means in mechanical engagement with said second clutch half and having an inoperative position holding the second clutch half uncoupled from said first clutch half and an inoperative position moving said second clutch half into engagement with said first clutch half for driving said guide means, electric switch means electrically connected to said actuating means, a control lever which is mechanically engageable with said switch means and which is operative to be moved responsive to an increase in the tension in the warp yarn being fed from said yarn supply means for thereby actuating said switch means to close and energize said electromagnetic actuating means, and mechanical biasing means uring said control lever to a position disengaged from said switch means.
 5. A warp tension control mechanism as claimed in claim 1, in which said brake torque varying means comprises a slider which carries thereon said fulcrum member, an internally screw-threaded portion forming part of said slider, an externally screw-threaded guide rod engaging said internally screw-threaded portion and engaging substantially in parallel to a direction in which said guide lever assumes when it is in said neutral position aligned with the axis of rotation of said brake wheel, clutch means having a first clutch half which is rotatable with and constantly driven from said yarn supply means and a second clutch half engageable with said clutch half and drivingly connected to said guide rod, said guide rod and said second clutch half being driven to rotate about their axes when the clutch halves are coupled together, electromagnetic actuating means in mechanical engagement with said second clutch half and having an operative position holding said second clutch half uncoupled from said first clutch half and an inoperative position moving the second clutch half into engagement with said first clutch half for driving said guide rod from said yarn supply means through said first clutch half, electric switch means electrically connected to said actuating means, a control lever which is mechanically engageable with said switch means and which is operative to be moved responsive to an increase in the tension in the warp yarn being fed from said yarn supply means for thereby actuating said switch means to close and energize said electromagnetic actuating means, and mechanical biasing means urging said control lever toward a position disengaged from said switch means.
 6. A warp tension control mechanism as claimed in claim 5, in which said first and second mechanical biasing means comprise first and second tension springs, the first tension spring being anchored at one end to an end portion of said guide lever close to said brake wheel and at the other end to said first brake arm and the second tension spring being anchored at one end to said end portion of the guide lever and at the other to said second brake arm.
 7. A warp tension control mechanism as claimed in claim 1, in which said brake shoes carried on said brake frames are located substantially in symmetry with respect to a line joining a center of said brake shoe carried on said second brake arm and the axis of rotation of said brake wheel.
 8. A warp tension control mechanism as claimed in claim 7, in which said at least three brake shoes are located substantially equiangularly around said axis of rotation of said brake wheel. 